As a cell phone has become increasingly functionalized and a memory card and file market has been enlarged in use recently, a flash memory is required to be large in capacity and low in cost. Thus, in order to implement the large capacity and low cost, a device having a small effective cell area making use of multilevel type and a virtual ground memory cell array type is being developed one after another. Especially, when the virtual ground memory cell array type is employed, since the area of individual memory cell can be reduced by devising a circuit, a device having a relatively small chip area can be implemented in the same process. However, when the virtual ground memory cell array type is used, since a leak current from an adjacent memory cell is inevitable in a reading operation, various inventive approaches are to be taken to implement high-speed and high-precision reading.
As a flash memory making use of the virtual ground memory cell array, a virtual ground memory cell array shown in FIG. 6 has been proposed (refer to non-patent document 1, for example). A reading method of the virtual ground type memory cell array will be described. When a memory cell 2 is selected and read, first, a read voltage Vcc is applied to BLSELn to select a segment including the selected memory cell 2. Then, 5V is applied to a word line WLi+1 and 0V is applied to other word lines. Then, a read voltage 1V is applied to the bit line BLk and other bit lines BL(k+1), BL(K+2), . . . are grounded to 0V. At this time, the read voltage Vcc is applied to SELn′ to apply the read voltage to a drain of the cell 2 to be read and 0V is applied to SELn. Thus, since the read voltage 1V is applied to the drain of the selected memory cell 2 to be read and a source is grounded to 0V, a current flowing in the selected memory cell 2 is detected by a sense amplifier and the reading can be performed.
Non-patent document 1: Anirban Roy et al., “A NEW FLASH ARCHITECTURE A 5.8 λ2 SCALABLE AMG FLASH CELL”, Symposium on VLSI Technology Digest of Technical Papers, P67, 1997